A method and a device for controlling a fuel pump are shown in German Published Patent Application No. 35 40 811. The system shown uses a solenoid valve for controlling the fuel pump for a diesel or gasoline engine. A pump piston is driven by a camshaft within a pump working space and, thus, pressurizes the fuel in the space. The fuel is then pumped to the individual cylinders of the engine by means of a fuel line. A solenoid valve is located between a fuel supply tank and the pump working space. An electronic control unit transmits control pulses to the solenoid valve. The solenoid valve opens and closes in response to the control pulses.
Based upon the circuit state of the solenoid valve, the pump piston delivers fuel into the combustion chambers of the engine. Drive pulses determine the point in time marking the beginning of the injection of fuel, and based upon the point in time that the injection of fuel is completed, the volume of fuel injected can be determined. Thus, with this system, it is not necessary to use metering groves, for example, to make a mechanical quantitative determination of the amount of fuel injected.
To establish the drive pulses, an increment wheel is mounted on the camshaft. After a synchronizing pulse appears, a counter is started, which counts the pulses on the increment wheel. After a specified number of pulses, the control system transmits a drive pulse to the solenoid valve. The drive pulse therefore defines the beginning of the injection of fuel. Subsequent counting of the increment pulses establishes the end of the injection of fuel.
One disadvantage of this device is that the meter-in flow control is relatively inaccurate. Since the drive pulses are established by counting the pulses of the increment wheel, the meter-in accuracy depends on the fineness of the increment wheel. Thus, both the beginning of the fuel flow output as well as the end of the fuel flow output are determined inaccurately. Due to limitations in manufacturing tolerances, only a finite number of teeth can be formed on the increment wheel. The pulses of the increment wheel are therefore spaced relatively far apart. This type of meter-in flow control is, accordingly, very inaccurate.
German Published Patent Application No. 35 40 313 which corresponds to U.S. Pat. No. 4,653,454, shows a method wherein the exact beginning of the fuel flow output can be established by using a solenoid valve mounted between the pump working space and the fuel supply. Mechanical components are used to determine the end of the injection of fuel and, consequently, the volume of fuel injected. The exact drive pulse indicating the beginning of the fuel injection is calculated in a manner similar to that described in German Published Patent Application No. 35 40 811. Proceeding from a synchronous pulse, the teeth on an increment wheel are counted. If the injection begins between two pulses of the increment wheel, then the remainder is interpolated therefrom. The interpolation is based upon a rotational frequency value averaged over several working cycles.
One problem with this system is that the rotational frequency can fluctuate over a single working cycle of the engine. The rotational frequency can also fluctuate from one working cycle to another. If an average rotational frequency value is used, as shown in German Published Patent Application No. 35 40 313, then the interpolation is likely to be very inaccurate due to changes in the rotational frequency during the meter-in flow control. The system shown in German Published Patent Application No. 35 40 313 attempts to compensate for these difficulties by using a correction factor which is dependent upon a characteristic map of performance data.
Even this correction factor, however, does not furnish sufficiently accurate values for indicating the beginning of the injection of fuel. Also, because the end of the injection of fuel is determined by mechanical components, an error occurring during the beginning of the fuel flow output can cause additional quantitative errors.
It is an object of the present invention, therefore, to provide a method and apparatus for controlling a fuel pump for a gasoline, diesel, or other type of internal combustion engine which overcomes the problems of such known methods and apparatus, and which accurately specifies both the beginning and the end of the injection of fuel.